package fs

import (
	"flag"
	"fmt"
	"os"
	"sync/atomic"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/metrics"
)

var disableMmap = flag.Bool("fs.disableMmap", is32BitPtr, "Whether to use pread() instead of mmap() for reading data files. "+
	"By default mmap() is used for 64-bit arches and pread() is used for 32-bit arches, since they cannot read data files bigger than 2^32 bytes in memory. "+
	"mmap() is usually faster for reading small data chunks than pread()")

// Disable mmap for architectures with 32-bit pointers in order to be able to work with files exceeding 2^32 bytes.
const is32BitPtr = (^uintptr(0) >> 32) == 0

// MustReadAtCloser is rand-access read interface.
type MustReadAtCloser interface {
	// MustReadAt must read len(p) bytes from offset off to p.
	MustReadAt(p []byte, off int64)

	// MustClose must close the reader.
	MustClose()
}

// ReaderAt implements rand-access reader.
type ReaderAt struct {
	readCalls uint64
	readBytes uint64

	f        *os.File
	mmapData []byte

	useLocalStats bool
}

// MustReadAt reads len(p) bytes at off from r.
func (r *ReaderAt) MustReadAt(p []byte, off int64) {
	if len(p) == 0 {
		return
	}
	if off < 0 {
		logger.Panicf("BUG: off=%d cannot be negative", off)
	}
	if len(r.mmapData) == 0 {
		n, err := r.f.ReadAt(p, off)
		if err != nil {
			logger.Panicf("FATAL: cannot read %d bytes at offset %d of file %q: %s", len(p), off, r.f.Name(), err)
		}
		if n != len(p) {
			logger.Panicf("FATAL: unexpected number of bytes read from file %q; got %d; want %d", r.f.Name(), n, len(p))
		}
	} else {
		if off > int64(len(r.mmapData)-len(p)) {
			logger.Panicf("BUG: off=%d is out of allowed range [0...%d] for len(p)=%d", off, len(r.mmapData)-len(p), len(p))
		}
		src := r.mmapData[off:]
		// The copy() below may result in thread block as described at https://valyala.medium.com/mmap-in-go-considered-harmful-d92a25cb161d .
		// But production workload proved this is OK in most cases, so use it without fear :)
		copy(p, src)
	}
	if r.useLocalStats {
		atomic.AddUint64(&r.readCalls, 1)
		atomic.AddUint64(&r.readBytes, uint64(len(p)))
	} else {
		readCalls.Inc()
		readBytes.Add(len(p))
	}
}

// MustClose closes r.
func (r *ReaderAt) MustClose() {
	fname := r.f.Name()
	if len(r.mmapData) > 0 {
		if err := mUnmap(r.mmapData[:cap(r.mmapData)]); err != nil {
			logger.Panicf("FATAL: cannot unmap data for file %q: %s", fname, err)
		}
		r.mmapData = nil
	}
	MustClose(r.f)
	r.f = nil

	if r.useLocalStats {
		readCalls.Add(int(r.readCalls))
		readBytes.Add(int(r.readBytes))
		r.readCalls = 0
		r.readBytes = 0
		r.useLocalStats = false
	}
	readersCount.Dec()
}

// SetUseLocalStats switches to local stats collection instead of global stats collection.
//
// This function must be called before the first call to MustReadAt().
//
// Collecting local stats may improve performance on systems with big number of CPU cores,
// since the locally collected stats is pushed to global stats only at MustClose() call
// instead of pushing it at every MustReadAt call.
func (r *ReaderAt) SetUseLocalStats() {
	r.useLocalStats = true
}

// MustFadviseSequentialRead hints the OS that f is read mostly sequentially.
//
// if prefetch is set, then the OS is hinted to prefetch f data.
func (r *ReaderAt) MustFadviseSequentialRead(prefetch bool) {
	if err := fadviseSequentialRead(r.f, prefetch); err != nil {
		logger.Panicf("FATAL: error in fadviseSequentialRead(%q, %v): %s", r.f.Name(), prefetch, err)
	}
}

// MustOpenReaderAt opens ReaderAt for reading from filename.
//
// MustClose must be called on the returned ReaderAt when it is no longer needed.
func MustOpenReaderAt(path string) *ReaderAt {
	f, err := os.Open(path)
	if err != nil {
		logger.Panicf("FATAL: cannot open file for reading: %s", err)
	}
	var r ReaderAt
	r.f = f
	if !*disableMmap {
		fi, err := f.Stat()
		if err != nil {
			MustClose(f)
			logger.Panicf("FATAL: error in fstat(%q): %s", path, err)
		}
		size := fi.Size()
		data, err := mmapFile(f, size)
		if err != nil {
			MustClose(f)
			logger.Panicf("FATAL: cannot mmap %q: %s", path, err)
		}
		r.mmapData = data
	}
	readersCount.Inc()
	return &r
}

var (
	readCalls    = metrics.NewCounter(`vm_fs_read_calls_total`)
	readBytes    = metrics.NewCounter(`vm_fs_read_bytes_total`)
	readersCount = metrics.NewCounter(`vm_fs_readers`)
)

func mmapFile(f *os.File, size int64) ([]byte, error) {
	if size == 0 {
		return nil, nil
	}
	if size < 0 {
		return nil, fmt.Errorf("got negative file size: %d bytes", size)
	}
	if int64(int(size)) != size {
		return nil, fmt.Errorf("file is too big to be mmap'ed: %d bytes", size)
	}
	// Round size to multiple of 4KB pages as `man 2 mmap` recommends.
	// This may help preventing SIGBUS panic at https://github.com/VictoriaMetrics/VictoriaMetrics/issues/581
	// The SIGBUS could occur if standard copy(dst, src) function may read beyond src bounds.
	sizeOrig := size
	if size%4096 != 0 {
		size += 4096 - size%4096
	}
	data, err := mmap(int(f.Fd()), int(size))
	if err != nil {
		return nil, fmt.Errorf("cannot mmap file with size %d: %w", size, err)
	}
	return data[:sizeOrig], nil
}